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the smallness of their mass and the eccentricity of their 
orbital paths. It is in these ever present points of dissimi- 
larity that I apprehend we shall find the cause of those 
visible, those varying, and those incidental differences from 
the planets, with which the term comet has become insepar- 
ably associated. It has not been observed that the smallest 
comets are most remarkable for their phenomena or their 
aspects. On the contrary the larger bodies of the class have 
always presented the most striking appearances, whence I 
infer that though these appearances are beheld only in con- 
nection with bodies of comparatively trivial mass, yet that 
insignificance of mass is not the primary element in the 
formation of the phenomena under consideration. The 
eccentricity of their orbits however having been a noticeable 
feature in connection with all the most remarkable comets, 
it is in this particular and the circumstances which accom- 
pany it, that I think the clue will be found to a solution of 
the enigma of their aspects. The most obvious difference 
from the planets which we might expect in the case of a 
comet on account of the smallness of its mass would be the 
feeble coercion of the elastic power of its gaseous parts and 
the consequent voluminous development of its atmosphere, 
whilst the eccentricity of its orbit would undoubtedly give 
rise to enormous changes in temperature of the particles 
composing it. It is in this extension of atmosphere and 
in the suddenness and violence of these thermal changes 
that I think it possible to find an explanation of almost 
every one of those appearances which are peculiar to comets 
as the ordinary and every day phenomena of their meteor- 
ology. 
Suppose for instance we have a planetary body composed 
